There is, in a number of industries and applications, a major need for a fault current fusing resistor that is very fast opening for relatively high currents and high AC and DC voltages. There is an even greater need for such a fault current fusing resistor that is inexpensive to manufacture and small in size, and that is characterized by a high degree of safety.
To state one example, there are power system applications where power-semiconductors (transistors, thyristors, SCRs etc.) are used in circuits which manage large currents at relatively high DC voltages. An illustration is the power drivers for motors such as are used on electric trains. The power-semiconductors associated with the control circuits or drive circuits occasionally short internally, which can cause the portion of the circuit that is in the short circuit path caused by the shorted power-semiconductor to be exposed suddenly to a very high fault current and fault voltage.
The preceding paragraph sets forth one example of a fault current condition, which is not a gradual or progressive current buildup to an excessive value, but instead a sudden large step or jump in current from normal to excessive. "Normal" is the current level present in the portion of the power-semiconductor circuit to be protected (potential short circuit path) against fault current during normal operation; it is a low current typically from a few milliamps to 2 amps. "Excessive" is what is present in the short circuit current path substantially immediately upon occurrence of the short, being the high fault current that is typically in excess of 15 amps, and more typically 50 amps to 500 amps or greater-with a voltage typically in excess of 125 volts up to 1,000 volts or higher.
It is badly needed to have an economical, fast-acting, fusing device that operates at those and other relatively high (excessive) fault currents and high voltages. Fast operation would effectively protect circuit board traces, and components, in the short circuit current path.
Insofar as applicant is presently informed, fault current fusing devices which operate at relatively high currents and that can interrupt at relatively high voltages are quite large, and/or expensive, and/or slow-acting, and/or have other disadvantages.